Hamstring strain injuries (HSIs) can be divided into two distinct types based on the action being performed when the injury occurs: ‘stretching-type’ and ‘sprinting-type’ injuries [1, 2]. Sprinting-type hamstring injuries are the most common in football, with the biceps femoris (BF) muscle most frequently injured in this muscle group (80% of all cases) . HSIs in football account for 12-16% of all injuries, and result in 5-6 HSIs per club per season. On average, this equates to missed playing time of 90 days and 15-21 matches per club per season [4, 5].
Activities involving large, fast hamstring strains are associated with the greatest number of injuries. In the last few decades great emphasis has been placed on improving the efficiency of injury prevention (e.g. FIFA 11+ program) . In spite of this, the number of hamstring injuries has not decreased . This is likely due to the fact that the mechanisms of HSI are still not clear.
Several risk factors have been identified in relation to HSI. For example, 24 seems to be a cut-off age beyond which injury risk increases . Reduced hamstring flexibility, hamstring muscle weakness, low hamstring-to-quadriceps strength ratio, and a shift in the optimum knee joint angle for hamstring force production have also been reported to increase HSI risk . Fatigue is another risk factor, as injuries occur more frequently at the ends of either half of a football match compared to earlier parts . Anatomical characteristics of the BF may also alter force distribution and injury risk .
Unfortunately, the recurrence of HSI is very high: up to 31% of injured muscles will sustain additional strain injuries . Recent findings also suggest that HSI may increase the likelihood of sustaining anterior cruciate ligament (ACL) injury, as the hamstring – which is the agonist of the ACL – has a decreased capacity to unload the ligament . Previous findings suggest that the activity of the muscle is decreased after HSI (even after return to sport) during lengthening contractions (when HSIs occur), and this apparent neural inhibition may in turn contribute to the high number of re-injuries [15, 16].
Previous studies have used magnetic resonance imaging (MRI) to examine hamstring length changes during simple movements such as controlled lengthening and shortening contractions . Results suggest that hamstring muscles actively lengthen (contract eccentrically) during hip flexion/knee extension, with larger strains observed in individuals with a previous HSI. Similarly, musculoskeletal models of running predict that muscle fibres contract eccentrically during the late swing phase, with strain magnitude increasing with running speed [18, 19]. In animals, larger muscle strains have been linked with greater injury risk. Thus, if the results from humans are correct, muscle fibre eccentric strain is a potential cause of HSI during high speed movements.
Importantly, both MRI and modelling studies have major inherent limitations. MRI provides direct tissue imaging but is limited to single plane movements and cannot be used in real-life dynamic conditions, whereas modelling studies depend upon generalized inputs, and do not directly examine tissue behaviour. Thus, modelled estimates of muscle behaviour are likely to produce inaccurate results for different individuals, especially when factors such as previous injury and muscle strength differ.
Ultrasound evaluation of the BF muscle is a valid  and reliable  method. A recent study  found that the fascicles of a previously injured BF muscle are shorter compared to the uninjured BF. This interesting study examined the muscle architecture in isometric conditions (without joint angular displacement) but not in real-life dynamic situations.
Is there a solution in sight? – Research at the University of Jyväskylä
Recently, ultrasound has been used extensively to study calf and quadriceps behaviour in real-life, dynamic tasks. Based on pilot studies in our research group, this method can also be used to study BF muscle behaviour dynamically, and thus to determine whether large and/or fast muscle strains are a potential mechanism for HSI. Moreover, by studying multiple risk factors and relating them to muscle mechanical behaviour in real-life movements, it may be possible to identify the actual mechanism(s) of HSI. This information could in turn be used to design more effective interventions to prevent HSIs.
Our current work aims to identify the mechanism(s) associated with hamstring injury and the relative importance of different risk factors. We are currently recruiting football players who would like to take part in this study as a subject. Players who participate in this study will get information about the functional state of their hamstrings, and will help make an important contribution to our understanding of hamstring injuries and effective injury prevention.
What kind of tests will I participate in?
The study will include a hamstring flexibility test, thigh strength tests, running, kicking, Nordic Hamstring Exercise, and a soccer-specific running protocol that is used in the English Premier League in rehabilitation and player testing. The testing will include state-of-the-art methods: leg muscle activity will be measured using electromyography (EMG), joint angles will be recorded using VICON motion analysis and hamstring muscle fascicle behavior will be measured with ultrasonography. Strength tests will be performed in a custom-made dynamometer, and hamstring muscle stimulation will be used before and after the multidirectional running test.
You can find some images and videos of the measurements from the following link:
Where does the measurement take place?
Neuromuscular Research Center, University of Jyväskylä, Viveca building, Rautpohjankatu 8, 40500 Jyväskylä, Finland
How long is the measurement?
The measurement will be done on 2 separate days, with 4-7 days in between. The first measurement takes around 2 hours; the second measurement takes approx. 3 hours. The schedule is flexible and can be arranged to suit your timetable.
Can I be a subject?
To participate you should meet the following criteria:
- 18-35 year old healthy male
- you have at least 3 years of experience in competitive football
- your preferred kicking leg is the right leg
- you did not have any other injuries than hamstring strain in the past 2 years
Players with previous hamstring strain injuries within the last 2 years are also encouraged to volunteer for the study.
Who should I contact to take part in this study?
Please send me an e-mail to make appointments for the measurements. I am also happy to answer any questions you may have (András Hegyi, email@example.com). The research team is led by Dr. Neil Cronin and Professor Taija Juutinen.
András Hegyi, PhD student in biomechanics
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